Linear actuators for opening, closing or throttling a valve are known. One known embodiment of a linear actuator comprises the following main components: a motor, reduction gearing, a thrust unit, torque sensors, position indicating switches and a coupling to the control element, e.g., a valve. The thrust unit comprises a drive sleeve coupled to the motor via the reduction gearing, and a ball screw which converts the rotation of the drive sleeve into linear travel of a ball screw. The ball screw comprises a nut on which the drive sleeve is mounted and a screw which forms the upper part of the ball screw. The end of the ball screw is coupled to the valve stem via the coupling. An anti-rotation device prevents rotation of the ball screw during rotation of the drive sleeve. Other linear actuators have a threaded stem/stem nut arrangement in place of a ball screw.
In accordance with the principle of operation of these prior art linear actuators, the valve has a fully closed state, a fully open state and a multiplicity of partially open states therebetween corresponding to various degrees of throttling. The actuator has a multiplicity of position indicating switches which are triggered in sequence as the traveling ball screw arrives at the respective linear positions corresponding to the fully open state and the partially open states of the valve. These position indicating switches output signals which can be used to turn off the drive motor when a desired valve state, other than the fully closed state, is attained or to provide valve position indication.
To ensure full closure of the valve, the valve disk must be moved into contact with the valve seat with a certain minimum force. As soon as the valve is securely closed, the drive motor is turned off. This is accomplished using a torque measuring device which senses a predetermined increase in the torque in the motor drive shaft caused by stoppage of the ball screw when the valve disk engages the valve seat. The signal output by the torque measuring device in response to a predetermined level of torque is then used to turn off the motor. The built-in electromechanical brake or locking gear set locks the actuator position when the power supply is switched off.
Linear actuators in the above-described class are prone to unreliable and unpredictable operation which results in unrepeatable closing force. Thus, there is a need for linear actuators having improved reliability.